The present invention relates to a critical power enhancement system for a pressurized fuel channel type nuclear reactor by reducing the fuel channel""s hydraulic resistance and thereby improving the critical heat flux of the fuel bundles.
The CANDU(copyright) reactor is an example of a pressurized fuel channel type nuclear reactor. It contains about 400 horizontally oriented pressure tubes each defining a fuel channel. Each fuel channel contains a plurality of fuel bundles longitudinally disposed in end-to-end relation within the pressure tube. Each fuel bundle comprises a plurality of elongated fuel rods containing fissionable material. The fuel rods are retained in parallel spaced relation uniformly about a central longitudinal axis between transversely disposed end plates. The end plates are of an open web design having apertures there through. High-pressure heavy or light water coolant, enters the fuel channel at one end, flows through the fuel bundles passing through the end plates and the spaces between the fuel rods so as to cool the fuel rods and remove heat produced by the fission process, and exits from the fuel channel at the other end. This heat is subsequently transferred by the coolant to a heat exchanger which produces steam that drives a turbine to produce electrical energy. The water flowing through the fuels bundles is pressurized and does not boil significantly.
The maximum power that can be produced within a fuel channel is determined by the maximum power that can be produced safely by individual fuel bundles within the fuel channel. The maximum power that can be produced within the fuel channel is normally referred to as the Critical Channel Power or CCP. The maximum power that can be produced safely in any given fuel bundle within that channel is called the Critical Bundle Power, and it is determined by variation in power production within the bundle, the corresponding local coolant conditions, and the design of the fuel bundle. The Critical Bundle Power is the power corresponding to the onset of a significant decrease in the efficiency of heat transfer from the bundle to the coolant, and the local heat flux at which this happens is referred to as the Critical Heat Flux or CHF. Since the high temperatures that can occur when the CHF is exceeded may damage the fuel bundle, the channel power and flow conditions are set to ensure that the CHF is never exceeded in any bundle.
Mechanistically, CHF occurs on a heated fuel element when some part of its surface can no longer be continuously wetted by the liquid coolant. There are two possible mechanisms leading to CHF: (i) breakdown of the liquid film, or xe2x80x9cdryoutxe2x80x9d on the fuel element sheath surface; or (ii) coalescence of bubbles near the fuel element sheath surface to form a vapour film. The actual mechanism depends upon the thermohydraulic conditions of the coolant surrounding the fuel element.
In order to ensure that the CHF is never exceeded in any bundle, a safety factor or operating margin is applied to the CCP, which in turn results in the power that can be produced by the reactor being reduced by approximately the same factor. If, however, the CCP could be increased, the power that could be produced by the reactor could also be increased. A similar situation apples to other types of water-cooled reactors.
In a given reactor, the pressure in the fuel channel is controlled by the reactor outlet header pressure and the enthalpy within the channel is controlled by the inlet header temperature. These values have been optimized and normally do not change. Hence, the CCP is primarily a function of the channel flow. Most of the known methods for improving the CCP seek to enhance CHF by adding turbulence inducing devices to selected locations within the fuel bundles. One example of the use of such devices is described in U.S. Pat. No. 5,493,590 issued to Atomic Energy of Canada Limited on Feb. 20, 1996. Such methods often achieve enhanced CHF at the expense of an increase in hydraulic resistance within the fuel channel. As discussed below, an increase in hydraulic resistance in the fuel channel causes the coolant flow to decrease, causing the CHF to occur at a lower fuel channel power. The resulting CCP is either worse than the case without the CHF enhancement devices, or is only marginally improved. High hydraulic resistance may also reduce the coolant flow through the fuel channels in an existing reactor that was not designed to accommodate a large pressure-drop resulting from such a large hydraulic resistance, thus affecting the overall performance of the reactor. In addition, turbulence enhancing devices require mechanical changes to the fuel bundle and can require corresponding change to the fuelling system and fuel handling apparatus of pressurized fuel channel type reactors, which is undesirable.
The present invention provides a critical power enhancement system for a pressurized fuel channel type nuclear reactor, which improves the critical heat flux of the fuel bundles by reducing the hydraulic resistance in the fuel channel.
In accordance with one aspect of the present invention, there is provided a fuel bundle pair assembly for use in a pressurized water-cooled nuclear reactor of the type adapted to be refuelled on-line (that is while operating at full power) by the insertion and removal of fuel bundles into and from a plurality of pressure tubes, said fuel bundle pair assembly comprising a pair of fuel bundles in end-to-end relation, each fuel bundle comprising a plurality of elongated fuel elements retained in parallel spaced relation uniformly about a longitudinal axis between transversely disposed end-plates, said end plates having an open web structure with apertures there through to permit coolant flow through said fuel channels in contact with said fuel elements, means for interlocking said pair of fuel bundles so as to maintain said fuel elements in a predetermined position of relative rotational alignment about said longitudinal axis and prevent axial separation of said pair of fuel bundles, said fuel bundle pair assembly being axially separable from adjacent bundles in a pressure tube to permit independent loading or unloading of said fuel bundle pair assembly.
In accordance with another aspect of the invention, there is provided a fuel channel assembly for use in a fuel-channel-type nuclear reactor of the type adapted to be refuelled on-line by the insertion and removal of fuel bundles into and from a plurality of fuel channel assemblies, each of said fuel channel assemblies comprising an elongated pressure tube and a plurality of fuel bundles longitudinally disposed in said pressure tube in end-to-end relation, each of said fuel bundles comprising a plurality of elongated fuel elements retained in parallel spaced relation uniformly about a longitudinal axis between transversely disposed end-plates, said end plates having apertures there through to permit coolant flow through said fuel channels in contact with said fuel elements, the fuel channel assembly further comprising at least one fuel bundle pair assembly, said fuel bundle pair assembly comprising a pair of fuel bundles in end-to-end relation and means for interlocking said pair of fuel bundles so as to maintain said fuel elements in a predetermined position of relative rotational alignment about said longitudinal axis and prevent axial separation of said pair of fuel bundles, said fuel bundle pair assembly being axially separable from adjacent bundles in the pressure tube to permit independent loading or unloading of said fuel bundle pair assembly.
In accordance with another aspect of the invention, there is provided a method of increasing the Critical Channel Power (CCP) in a pressurized fuel-channel-type nuclear reactor of the type adapted to be refuelled on-line by the insertion and removal of fuel bundles into and from a plurality fuel channel assemblies, each of said fuel channel assemblies comprising an elongated pressure tube and a plurality of fuel bundles longitudinally disposed in said pressure tube in end-to-end relation, each of said fuel bundles comprising a plurality of elongated fuel elements retained in parallel spaced relation uniformly about a longitudinal axis between transversely disposed end-plates, said end plates having apertures there through to permit coolant flow through said fuel channels in contact with said fuel elements, the method comprising the steps of interlocking the facing end-plates of a pair of fuel bundles in end-to-end relation to maintain a predetermined position of relative rotational alignment about said longitudinal axis and prevent axial separation of said pair of fuel bundles, inserting said interlocked pair of fuel bundles into a fuel channel, and removing two unpaired fuel bundles from said fuel channel.
In accordance with another aspect of the invention, there is provided a fuel bundle end-plate for retaining a plurality of elongated fuel elements in parallel spaced relation uniformly about a longitudinal axis, said end plate having an open web structure with apertures therethrough to permit coolant flow through said fuel channels in contact with said fuel elements, said end plate comprising inner, intermediate and outer concentric ring web members, said inner and intermediate ring web members being interconnected by inner cross-webs and said intermediate and outer ring web members being interconnected by outer cross-webs and comprising two hook members each connected to a cross-web and having a first leg portion projecting longitudinally and a second leg portion extending transversely, said first and second leg portions together with the cross-web forming a recess adapted to closely receive the corresponding cross-web of a facing end-plate.